The X-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. Typical X-ray tubes are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor consisting of a cylindrical rotor built into a cantilevered axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the X-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to ground. The X-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode.
In an X-ray tube device with a rotatable anode, the target has previously consisted of a disk made of a refractory metal such as tungsten, and the X-rays are generated by making the electron beam collide with this target, while the target is being rotated at high speed. Rotation of the target is achieved by driving the rotor provided on a support shaft extending from the target. Such an arrangement is typical of rotating X-ray tubes and has remained relatively unchanged in concept of operation since its introduction.
However, the operating conditions for X-ray tubes have changed considerably in the last two decades. Due to continuous demands from radiologists for higher power from X-ray tubes, more and more tubes are using composite rotating anodes with tungsten-rhenium as a layer, molybdenum alloys as a substrate, and brazed graphite as a heat sink. Application of the inertia welding process, disclosed in U.S. Pat. No. 5,592,525, totally incorporated herein by reference, helps to eliminate problems associated with attachment of the stem to the target; however, higher power applied to the target leads to more heat traveling to the lower portion of the integral target-stem assembly and hence to the bearing, which adversely affects tube life. For some applications, the temperature is so high that stresses existing between the target flange and the mating thermobarrier lead to plastic deformation of TZM shaft. This TZM shaft does not have sufficient strength due to recrystallization which occurs during the brazing of graphite to the target cap at 1600-1800 C. As a result of high temperature at the bottom portion and plastic deformation of the target shaft, unbalance takes place, which in turn leads to premature failure of the tube.
It would be desirable then to have an improved X-ray tube target assembly design with reduced heat transfer to overcome problems associated with prior art structures and for improving the service life of an X-ray tube target.